A gene map is a map showing the locations for genes on a chromosome. The locations at which chromosomes tend to cross over differ depending on the randomness of recombination or crossing over during prophase I of meiosis. To determine which genes will be crossing over most often, we can use the known frequency of crossing over events to determine the locations of genes on a map.
First, we need to know what crossing over is. Crossing over is the process by which genes are exchanged between homologous chromosomes during meiosis, the formation of germ cells. Unless two genes are linked, genes follow the law of independent assortment, where during meiosis, different alleles for different genes can be separated during the crossing over process.
From this information, we can infer correctly that the greater the distance between two genes, the more often there will be crossing-over events between them. In other words, the further two genes are apart from another on the gene map, the more often you will see two alleles on the same chromosome separated in the crossing over process.
Thus, the answer to your question is based purely on the distance between two genes. If gene A is on one end of the gene map on a chromosome and if gene B is on the other end, then these chromosomes have the highest likelihood of being separated during crossing over.